A spark plug is assembled to a combustion apparatus such as an internal combustion engine (an engine), and is used to ignite an air-fuel mixture or the like. Generally, the spark plug includes an insulator having an axial hole, a center electrode, and a metal shell main body. The axial hole extends in an axial direction. The center electrode is inserted into the tip end side of the axial hole. The metal shell main body is provided on the outer periphery of the insulator. A thread portion and a flange seat portion are formed on the outer peripheral surface of the metal shell. The thread portion is threadably mounted on a mounting hole of the combustion apparatus. The seat portion is positioned at a rear end side with respect to the thread portion, and projects radially outward.
Furthermore, a protrusion is formed on an inner peripheral surface of the metal shell at the tip end side with respect to the seat portion. The protrusion projects toward the inner peripheral side. A lock portion is disposed on the outer periphery of the insulator. The lock portion is locked on a locked surface of the protrusion directly or indirectly via a sheet packing and similar member. Additionally, at a rear end portion of the metal shell, a caulking portion is formed to be bent toward the inner peripheral side. The insulator is secured to the metal shell in a state held between the protrusion and the caulking portion (that is, in a state where an axial force is applied from the metal shell). Accordingly, the axial force applied to the insulator provides a sufficiently large contact pressure between the locked surface and the lock portion. As a result, this ensures good air tightness between the metal shell and the insulator.
In order to ensure excellent air tightness within the combustion chamber, a known technique includes a ring-shaped gasket disposed at a thread root, which is disposed at the rear end side of the thread portion. When the spark plug is mounted on the combustion apparatus, the gasket contacts the seating portion of the combustion apparatus. Additionally, one spark plug (what is called a conical seat type) is proposed to further improve the air tightness (for example, see JP-A-2011-103276). In this spark plug, the gasket is not provided, and the seat portion includes a tip end face as a tapering surface that is tapered off toward the tip end side in the axial direction. The tapering surface directly contacts the seating portion.
Additionally, a process (a caulking process) for forming the caulking portion is performed as follows to secure the insulator to the metal shell. That is, in a state where the insulator is inserted into the metal shell, a tip end portion of the metal shell is inserted into an insertion hole of a predetermined receiving die, thus holding the metal shell at the receiving die. At this time, the tapering surface contacts a tapered receiving surface, which connects to with an opening of the insertion hole and has the same slanted angle as a slanted angle of the tapering surface. Subsequently, an annular pressing die is used to apply a load to the rear end portion of the metal shell along the axial direction. Accordingly, the caulking portion is formed in the rear end portion of the metal shell, and the metal shell and the insulator are secured to each other. Note that a bulge portion is formed together with the caulking portion in the caulking process. The bulge portion is formed by deformation of a relatively thin portion positioned between the caulking portion and the seat portion in the metal shell, and projects toward the outer peripheral side. The formation of the bulge portion allows more surely applying the axial force to the insulator from the metal shell.
Now, in the conical seat type spark plug, when a load is applied to the metal shell in the caulking process, the seat portion and the protrusion may be deformed excessively. If excessive deformation occurs at the seat portion and the protrusion, the axial force applied to the insulator from the metal shell may be extremely decreased. As a result, this may cause decrease in air tightness between the metal shell and the insulator.
Even if the decrease in axial force can be reduced, when an area of the locked surface is excessively large compared with the size of the axial force, the contact pressure between the locked surface and the lock portion becomes low. Eventually, this may cause decrease in air tightness.
The present invention has been conceived to solve the above-mentioned problems. An advantage of the invention is a spark plug that more surely prevents deformation of a seat portion and a protrusion in a caulking process so as to ensure good air tightness between a metal shell and an insulator